diff --git a/internal/protocol/const.go b/internal/protocol/const.go
new file mode 100644
index 0000000..b4c396c
--- /dev/null
+++ b/internal/protocol/const.go
@@ -0,0 +1,94 @@
+package protocol
+
+// 协议常量与默认上限（与 DEVELOPMENT 第 6、11 节示例一致）。
+
+const (
+	Version = 1
+
+	TypeHello             = "hello"
+	TypeResp              = "resp"
+	TypeSend              = "send"
+	TypeMsg               = "msg"
+	TypeAck               = "ack"
+	TypeRecall            = "recall"
+	TypeStatus            = "status"
+	TypeReceipt           = "receipt"
+	TypeReceiptAck        = "receipt_ack"
+	TypeRevoked           = "revoked"
+	TypePresenceGet       = "presence.get"
+	TypeDirectoryList     = "directory.list"
+	TypePresenceWatch     = "presence.watch"
+	TypePresence          = "presence"
+	TypeUnlock            = "unlock"
+	TypeSelfGet           = "self.get"
+	TypeSelfUpdate        = "self.update"
+	TypeSelfTalkPassword  = "self.talk_password"
+	TypeSelfLoginPassword = "self.login_password"
+	TypeSelfLogout        = "self.logout"
+	TypeGroupCreate       = "group.create"
+	TypeGroupAdd          = "group.add"
+	TypeGroupRemove       = "group.remove"
+	TypeGroupLeave        = "group.leave"
+	TypeGroupTransfer     = "group.transfer"
+	TypeGroupRename       = "group.rename"
+	TypeGroupDissolve     = "group.dissolve"
+	TypeGroupList         = "group.list"
+	TypeGroupGet          = "group.get"
+	TypeGroupEvent        = "group_event"
+	TypeFatal             = "fatal"
+
+	TargetEndpoint = "endpoint"
+	TargetGroup    = "group"
+
+	EncUTF8   = "utf8"
+	EncBase64 = "base64"
+
+	DefaultContentTypeUTF8   = "text/plain; charset=utf-8"
+	DefaultContentTypeBase64 = "application/octet-stream"
+
+	SessionTokenPrefix = "nst_"
+
+	MinIDLen             = 1
+	MaxIDLen             = 64
+	MaxContentTypeLen    = 128
+	MaxNameChars         = 64
+	MinLoginPasswordLen  = 8
+	MaxLoginPasswordLen  = 128
+	MinTalkPasswordLen   = 4
+	MaxTalkPasswordLen   = 64
+	MinMaxReceiveBytes   = 1024
+	MaxPresenceGetIDs    = 200
+	MaxPresenceWatchIDs  = 1000
+	MaxPageLimit         = 200
+	DefaultOfflineTTL    = int64(86400)
+	DefaultMaxBodyBytes  = 262144
+	DefaultMaxMetaBytes  = 4096
+	DefaultMaxFrameBytes = 786432
+)
+
+// 第 6.10 节错误码。
+const (
+	CodeBadRequest              = "bad_request"
+	CodeNotReady                = "not_ready"
+	CodeUnauthorized            = "unauthorized"
+	CodeForbidden               = "forbidden"
+	CodeNotFound                = "not_found"
+	CodeInvalidTarget           = "invalid_target"
+	CodeConflict                = "conflict"
+	CodeIDTaken                 = "id_taken"
+	CodeBodyTooLarge            = "body_too_large"
+	CodeMetaTooLarge            = "meta_too_large"
+	CodeFrameTooLarge           = "frame_too_large"
+	CodeResponseTooLarge        = "response_too_large"
+	CodeTalkPasswordRequired    = "talk_password_required"
+	CodeTalkPasswordInvalid     = "talk_password_invalid"
+	CodeRateLimited             = "rate_limited"
+	CodeNotMember               = "not_member"
+	CodeOwnerCannotLeave        = "owner_cannot_leave"
+	CodeGroupFull               = "group_full"
+	CodeQuotaExceeded           = "quota_exceeded"
+	CodeEndpointDisabled        = "endpoint_disabled"
+	CodeRegistrationClosed      = "registration_closed"
+	CodeRegistrationCodeInvalid = "registration_code_invalid"
+	CodeBusy                    = "busy"
+)
diff --git a/internal/protocol/decode.go b/internal/protocol/decode.go
new file mode 100644
index 0000000..7116497
--- /dev/null
+++ b/internal/protocol/decode.go
@@ -0,0 +1,115 @@
+package protocol
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+type typePeek struct {
+	V    int    `json:"v"`
+	Type string `json:"type"`
+}
+
+// Decode 解析一帧 MQTT/应用 JSON，返回具体类型。
+func Decode(data []byte) (any, error) {
+	var peek typePeek
+	if err := Unmarshal(data, &peek); err != nil {
+		return nil, badRequest("invalid json")
+	}
+	if peek.V != 0 && peek.V != Version {
+		return nil, badRequest("unsupported version")
+	}
+	if peek.Type == "" {
+		return nil, badRequest("missing type")
+	}
+
+	var out any
+	switch peek.Type {
+	case TypeHello:
+		out = &Hello{}
+	case TypeResp:
+		out = &Resp{}
+	case TypeSend:
+		out = &Send{}
+	case TypeMsg:
+		out = &Msg{}
+	case TypeAck:
+		out = &Ack{}
+	case TypeRecall:
+		out = &Recall{}
+	case TypeStatus:
+		out = &Status{}
+	case TypeReceipt:
+		out = &Receipt{}
+	case TypeReceiptAck:
+		out = &ReceiptAck{}
+	case TypeRevoked:
+		out = &Revoked{}
+	case TypePresenceGet:
+		out = &PresenceGet{}
+	case TypeDirectoryList:
+		out = &DirectoryList{}
+	case TypePresenceWatch:
+		out = &PresenceWatch{}
+	case TypePresence:
+		out = &Presence{}
+	case TypeUnlock:
+		out = &Unlock{}
+	case TypeSelfGet:
+		out = &SelfGet{}
+	case TypeSelfUpdate:
+		out = &SelfUpdate{}
+	case TypeSelfTalkPassword:
+		out = &SelfTalkPassword{}
+	case TypeSelfLoginPassword:
+		out = &SelfLoginPassword{}
+	case TypeSelfLogout:
+		out = &SelfLogout{}
+	case TypeGroupCreate:
+		out = &GroupCreate{}
+	case TypeGroupAdd:
+		out = &GroupAdd{}
+	case TypeGroupRemove:
+		out = &GroupRemove{}
+	case TypeGroupLeave:
+		out = &GroupLeave{}
+	case TypeGroupTransfer:
+		out = &GroupTransfer{}
+	case TypeGroupRename:
+		out = &GroupRename{}
+	case TypeGroupDissolve:
+		out = &GroupDissolve{}
+	case TypeGroupList:
+		out = &GroupList{}
+	case TypeGroupGet:
+		out = &GroupGet{}
+	case TypeGroupEvent:
+		out = &GroupEvent{}
+	case TypeFatal:
+		out = &Fatal{}
+	default:
+		return nil, badRequest(fmt.Sprintf("unknown type %q", peek.Type))
+	}
+	if err := Unmarshal(data, out); err != nil {
+		return nil, badRequest("invalid json for type")
+	}
+	return out, nil
+}
+
+// DecodeRegister 解析注册 HTTP 请求体。
+func DecodeRegister(data []byte) (*RegisterRequest, error) {
+	var req RegisterRequest
+	if err := Unmarshal(data, &req); err != nil {
+		return nil, badRequest("invalid json")
+	}
+	return &req, nil
+}
+
+// MustRaw 将值编码为 json.RawMessage（用于填 Resp.Data）。
+func MustRaw(v any) json.RawMessage {
+	b, err := Marshal(v)
+	if err != nil {
+		panic(err)
+	}
+	return b
+}
diff --git a/internal/protocol/encode.go b/internal/protocol/encode.go
new file mode 100644
index 0000000..95b6e3f
--- /dev/null
+++ b/internal/protocol/encode.go
@@ -0,0 +1,48 @@
+package protocol
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+)
+
+// Marshal 将值编码为 UTF-8 JSON，不转义 HTML 与非 ASCII，且不含尾部换行。
+func Marshal(v any) ([]byte, error) {
+	var buf bytes.Buffer
+	if err := Encode(&buf, v); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// Encode 写入 JSON。json.Encoder 默认会追加 '\n'，这里去掉，保证整帧字节数与线上一致。
+func Encode(w io.Writer, v any) error {
+	var buf bytes.Buffer
+	enc := json.NewEncoder(&buf)
+	enc.SetEscapeHTML(false)
+	if err := enc.Encode(v); err != nil {
+		return err
+	}
+	b := buf.Bytes()
+	if n := len(b); n > 0 && b[n-1] == '\n' {
+		b = b[:n-1]
+	}
+	_, err := w.Write(b)
+	return err
+}
+
+// Unmarshal 解码 JSON。
+func Unmarshal(data []byte, v any) error {
+	dec := json.NewDecoder(bytes.NewReader(data))
+	dec.UseNumber()
+	return dec.Decode(v)
+}
+
+// FrameBytes 返回整帧编码后的字节数。
+func FrameBytes(v any) (int, error) {
+	b, err := Marshal(v)
+	if err != nil {
+		return 0, err
+	}
+	return len(b), nil
+}
diff --git a/internal/protocol/error.go b/internal/protocol/error.go
new file mode 100644
index 0000000..9c7c819
--- /dev/null
+++ b/internal/protocol/error.go
@@ -0,0 +1,46 @@
+package protocol
+
+// ErrorBody 是 resp / 注册失败里的 error 对象。
+type ErrorBody struct {
+	Code    string `json:"code"`
+	Message string `json:"message"`
+}
+
+// Error 是协议包校验失败时返回的错误，Code 对应第 6.10 节。
+type Error struct {
+	Code    string
+	Message string
+}
+
+func (e *Error) Error() string {
+	if e == nil {
+		return ""
+	}
+	if e.Message == "" {
+		return e.Code
+	}
+	return e.Code + ": " + e.Message
+}
+
+func (e *Error) Body() ErrorBody {
+	if e == nil {
+		return ErrorBody{}
+	}
+	return ErrorBody{Code: e.Code, Message: e.Message}
+}
+
+func badRequest(msg string) *Error {
+	return &Error{Code: CodeBadRequest, Message: msg}
+}
+
+func bodyTooLarge(msg string) *Error {
+	return &Error{Code: CodeBodyTooLarge, Message: msg}
+}
+
+func metaTooLarge(msg string) *Error {
+	return &Error{Code: CodeMetaTooLarge, Message: msg}
+}
+
+func frameTooLarge(msg string) *Error {
+	return &Error{Code: CodeFrameTooLarge, Message: msg}
+}
diff --git a/internal/protocol/fingerprint.go b/internal/protocol/fingerprint.go
new file mode 100644
index 0000000..f5eb284
--- /dev/null
+++ b/internal/protocol/fingerprint.go
@@ -0,0 +1,81 @@
+package protocol
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"encoding/hex"
+)
+
+// RequestFingerprint 计算第 7.3 节发送请求指纹（十六进制小写 SHA-256）。
+// 字段：to.kind、to.id、body.enc、content_type、解码后正文、meta（键排序）、
+// send_at_ms、delay_ms、offline.keep、offline.ttl_seconds、receipt。
+// 不含 talk_password 与 rid。meta 键顺序不影响结果。
+// receipt / offline 使用文档默认值后的有效值；缺省的 send_at_ms、delay_ms 不写入对应槽位。
+func RequestFingerprint(s *Send) (string, error) {
+	raw, err := DecodeBody(s.Body)
+	if err != nil {
+		return "", err
+	}
+	metaJSON, err := MetaCanonicalJSON(s.Meta)
+	if err != nil {
+		return "", badRequest("invalid meta")
+	}
+
+	h := sha256.New()
+	writeStr(h, s.To.Kind)
+	writeStr(h, s.To.ID)
+	writeStr(h, s.Body.Enc)
+	writeStr(h, EffectiveContentType(s.Body))
+	writeBytes(h, raw)
+	writeBytes(h, metaJSON)
+
+	writeOptInt64(h, s.SendAtMs)
+	writeOptInt64(h, s.DelayMs)
+
+	keep := EffectiveOfflineKeep(s)
+	writeBool(h, keep)
+	ttl := EffectiveOfflineTTL(s)
+	writeInt64(h, ttl)
+	writeBool(h, EffectiveReceipt(s))
+
+	sum := h.Sum(nil)
+	return hex.EncodeToString(sum), nil
+}
+
+type hashWriter interface {
+	Write(p []byte) (int, error)
+}
+
+func writeStr(h hashWriter, s string) {
+	writeBytes(h, []byte(s))
+}
+
+func writeBytes(h hashWriter, b []byte) {
+	var lenBuf [8]byte
+	binary.BigEndian.PutUint64(lenBuf[:], uint64(len(b)))
+	_, _ = h.Write(lenBuf[:])
+	_, _ = h.Write(b)
+}
+
+func writeBool(h hashWriter, v bool) {
+	if v {
+		_, _ = h.Write([]byte{1})
+	} else {
+		_, _ = h.Write([]byte{0})
+	}
+}
+
+func writeInt64(h hashWriter, v int64) {
+	var buf [8]byte
+	binary.BigEndian.PutUint64(buf[:], uint64(v))
+	_, _ = h.Write(buf[:])
+}
+
+func writeOptInt64(h hashWriter, v *int64) {
+	if v == nil {
+		_, _ = h.Write([]byte{0})
+		return
+	}
+	_, _ = h.Write([]byte{1})
+	writeInt64(h, *v)
+}
diff --git a/internal/protocol/ids.go b/internal/protocol/ids.go
new file mode 100644
index 0000000..37e3337
--- /dev/null
+++ b/internal/protocol/ids.go
@@ -0,0 +1,94 @@
+package protocol
+
+import (
+	"strings"
+	"unicode/utf8"
+)
+
+func isEndpointIDChar(c rune) bool {
+	return (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9') || c == '_' || c == '.' || c == '-'
+}
+
+func isMessageIDChar(c rune) bool {
+	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '_' || c == '.' || c == '-'
+}
+
+// ValidEndpointID 校验端编号或群编号（小写字母、数字、_ . -，1–64）。
+func ValidEndpointID(id string) bool {
+	n := len(id)
+	if n < MinIDLen || n > MaxIDLen {
+		return false
+	}
+	for _, c := range id {
+		if !isEndpointIDChar(c) {
+			return false
+		}
+	}
+	return true
+}
+
+// ValidMessageID 校验消息号（字母数字 _ . -，区分大小写，1–64）。
+func ValidMessageID(id string) bool {
+	n := len(id)
+	if n < MinIDLen || n > MaxIDLen {
+		return false
+	}
+	for _, c := range id {
+		if !isMessageIDChar(c) {
+			return false
+		}
+	}
+	return true
+}
+
+// ValidLoginPassword 校验登录密码：8–128 字符，且不能以 nst_ 开头。空串表示由服务器生成，视为合法。
+func ValidLoginPassword(pw string) bool {
+	if pw == "" {
+		return true
+	}
+	if strings.HasPrefix(pw, SessionTokenPrefix) {
+		return false
+	}
+	n := utf8.RuneCountInString(pw)
+	return n >= MinLoginPasswordLen && n <= MaxLoginPasswordLen
+}
+
+// LoginPasswordForbiddenPrefix 报告密码是否因 nst_ 前缀非法。
+func LoginPasswordForbiddenPrefix(pw string) bool {
+	return strings.HasPrefix(pw, SessionTokenPrefix)
+}
+
+// ValidTalkPassword 校验对话密码：空表示清除/不设；否则 4–64 字符。
+func ValidTalkPassword(pw string) bool {
+	if pw == "" {
+		return true
+	}
+	n := utf8.RuneCountInString(pw)
+	return n >= MinTalkPasswordLen && n <= MaxTalkPasswordLen
+}
+
+// ValidName 校验名称：最多 64 个 Unicode 字符。
+func ValidName(name string) bool {
+	return utf8.RuneCountInString(name) <= MaxNameChars
+}
+
+func requireRID(rid string) error {
+	if rid == "" {
+		return badRequest("missing rid")
+	}
+	return nil
+}
+
+func requireVersion(v int) error {
+	if v != Version {
+		return badRequest("unsupported version")
+	}
+	return nil
+}
+
+func requireType(got, want string) error {
+	if got != want {
+		return badRequest("wrong type")
+	}
+	return nil
+}
diff --git a/internal/protocol/protocol_test.go b/internal/protocol/protocol_test.go
new file mode 100644
index 0000000..82f0f23
--- /dev/null
+++ b/internal/protocol/protocol_test.go
@@ -0,0 +1,535 @@
+package protocol_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"git.asio.asia/nixevol/NixMsg/internal/protocol"
+)
+
+func ptrInt(v int) *int       { return &v }
+func ptrInt64(v int64) *int64 { return &v }
+func ptrBool(v bool) *bool    { return &v }
+
+func mustMarshal(t *testing.T, v any) []byte {
+	t.Helper()
+	b, err := protocol.Marshal(v)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	return b
+}
+
+func TestEncodeNoHTMLEscapeAndNoNewline(t *testing.T) {
+	msg := &protocol.Msg{
+		V:    protocol.Version,
+		Type: protocol.TypeMsg,
+		ID:   "id-1",
+		From: "app-1",
+		To:   protocol.Target{Kind: protocol.TargetEndpoint, ID: "device-1"},
+		Body: protocol.Body{Enc: protocol.EncUTF8, Data: "你好<script>&"},
+	}
+	b := mustMarshal(t, msg)
+	if bytes.Contains(b, []byte(`\u`)) {
+		t.Fatalf("unexpected unicode escape: %s", b)
+	}
+	if bytes.Contains(b, []byte(`\u003c`)) || !bytes.Contains(b, []byte("<script>")) {
+		t.Fatalf("HTML should not be escaped: %s", b)
+	}
+	if bytes.Contains(b, []byte("你好")) == false {
+		t.Fatalf("non-ASCII should remain: %s", b)
+	}
+	if len(b) == 0 || b[len(b)-1] == '\n' {
+		t.Fatalf("must not end with newline: %q", b)
+	}
+	n, err := protocol.FrameBytes(msg)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != len(b) {
+		t.Fatalf("FrameBytes=%d len=%d", n, len(b))
+	}
+}
+
+func TestDecodeRoundTripAllFrames(t *testing.T) {
+	lim := protocol.DefaultLimits()
+	cases := []struct {
+		name string
+		in   any
+		typ  string
+		val  func(any) error
+	}{
+		{
+			name: "hello",
+			in: &protocol.Hello{
+				V: protocol.Version, Type: protocol.TypeHello, RID: "1",
+				MaxReceiveBytes: ptrInt(4096), Client: "go-sdk/0.1",
+			},
+			typ: protocol.TypeHello,
+			val: func(v any) error { return v.(*protocol.Hello).Validate() },
+		},
+		{
+			name: "resp_ok",
+			in: &protocol.Resp{
+				V: protocol.Version, Type: protocol.TypeResp, RID: "1", OK: true,
+				Data: protocol.MustRaw(protocol.HelloData{
+					ServerTimeMs: 1, ServerVersion: "0.1.0",
+					MaxBodyBytes: 262144, MaxMetaBytes: 4096, MaxFrameBytes: 786432,
+					MaxTTLSeconds: 1, MaxScheduleSeconds: 1, AckTimeoutSeconds: 1,
+					SessionToken: "nst_abc",
+				}),
+			},
+			typ: protocol.TypeResp,
+			val: func(v any) error { return v.(*protocol.Resp).Validate() },
+		},
+		{
+			name: "resp_err",
+			in: &protocol.Resp{
+				V: protocol.Version, Type: protocol.TypeResp, RID: "1", OK: false,
+				Error: &protocol.ErrorBody{Code: protocol.CodeTalkPasswordRequired, Message: "需要对话密码"},
+			},
+			typ: protocol.TypeResp,
+			val: func(v any) error { return v.(*protocol.Resp).Validate() },
+		},
+		{
+			name: "send",
+			in: &protocol.Send{
+				V: protocol.Version, Type: protocol.TypeSend, RID: "2", ID: "018f-Ab",
+				To:   protocol.Target{Kind: protocol.TargetEndpoint, ID: "device-1"},
+				Body: protocol.Body{Enc: protocol.EncUTF8, Data: "hello"},
+				Meta: map[string]any{"k": "v"},
+			},
+			typ: protocol.TypeSend,
+			val: func(v any) error { return v.(*protocol.Send).Validate(lim) },
+		},
+		{
+			name: "msg",
+			in: &protocol.Msg{
+				V: protocol.Version, Type: protocol.TypeMsg, ID: "018f",
+				From: "app-1", To: protocol.Target{Kind: protocol.TargetGroup, ID: "g_ab12cd34"},
+				Body: protocol.Body{Enc: protocol.EncUTF8, Data: "hello"}, SendAtMs: 1,
+			},
+			typ: protocol.TypeMsg,
+			val: func(v any) error { return v.(*protocol.Msg).Validate(lim) },
+		},
+		{
+			name: "ack",
+			in:   &protocol.Ack{V: protocol.Version, Type: protocol.TypeAck, RID: "3", From: "app-1", ID: "018f"},
+			typ:  protocol.TypeAck,
+			val:  func(v any) error { return v.(*protocol.Ack).Validate() },
+		},
+		{
+			name: "recall",
+			in:   &protocol.Recall{V: protocol.Version, Type: protocol.TypeRecall, RID: "4", ID: "018f"},
+			typ:  protocol.TypeRecall,
+			val:  func(v any) error { return v.(*protocol.Recall).Validate() },
+		},
+		{
+			name: "status",
+			in:   &protocol.Status{V: protocol.Version, Type: protocol.TypeStatus, RID: "5", ID: "018f", Limit: 100},
+			typ:  protocol.TypeStatus,
+			val:  func(v any) error { return v.(*protocol.Status).Validate() },
+		},
+		{
+			name: "receipt",
+			in: &protocol.Receipt{
+				V: protocol.Version, Type: protocol.TypeReceipt, ReceiptID: "9001",
+				ID: "018f", EndpointID: "device-1", State: "accepted", AtMs: 1,
+			},
+			typ: protocol.TypeReceipt,
+			val: func(v any) error { return v.(*protocol.Receipt).Validate() },
+		},
+		{
+			name: "receipt_ack",
+			in:   &protocol.ReceiptAck{V: protocol.Version, Type: protocol.TypeReceiptAck, RID: "6", ReceiptID: "9001"},
+			typ:  protocol.TypeReceiptAck,
+			val:  func(v any) error { return v.(*protocol.ReceiptAck).Validate() },
+		},
+		{
+			name: "revoked",
+			in:   &protocol.Revoked{V: protocol.Version, Type: protocol.TypeRevoked, ID: "018f", From: "app-1", Reason: "recalled"},
+			typ:  protocol.TypeRevoked,
+			val:  func(v any) error { return v.(*protocol.Revoked).Validate() },
+		},
+		{
+			name: "presence.get",
+			in:   &protocol.PresenceGet{V: protocol.Version, Type: protocol.TypePresenceGet, RID: "7", IDs: []string{"a", "b"}},
+			typ:  protocol.TypePresenceGet,
+			val:  func(v any) error { return v.(*protocol.PresenceGet).Validate() },
+		},
+		{
+			name: "directory.list",
+			in:   &protocol.DirectoryList{V: protocol.Version, Type: protocol.TypeDirectoryList, RID: "8", Limit: 100},
+			typ:  protocol.TypeDirectoryList,
+			val:  func(v any) error { return v.(*protocol.DirectoryList).Validate() },
+		},
+		{
+			name: "presence.watch",
+			in:   &protocol.PresenceWatch{V: protocol.Version, Type: protocol.TypePresenceWatch, RID: "9", IDs: []string{"a"}, All: false},
+			typ:  protocol.TypePresenceWatch,
+			val:  func(v any) error { return v.(*protocol.PresenceWatch).Validate() },
+		},
+		{
+			name: "presence",
+			in:   &protocol.Presence{V: protocol.Version, Type: protocol.TypePresence, ID: "a", Online: true, AtMs: 1},
+			typ:  protocol.TypePresence,
+			val:  func(v any) error { return v.(*protocol.Presence).Validate() },
+		},
+		{
+			name: "unlock",
+			in:   &protocol.Unlock{V: protocol.Version, Type: protocol.TypeUnlock, RID: "10", EndpointID: "b", TalkPassword: "secret"},
+			typ:  protocol.TypeUnlock,
+			val:  func(v any) error { return v.(*protocol.Unlock).Validate() },
+		},
+		{
+			name: "self.get",
+			in:   &protocol.SelfGet{V: protocol.Version, Type: protocol.TypeSelfGet, RID: "11"},
+			typ:  protocol.TypeSelfGet,
+			val:  func(v any) error { return v.(*protocol.SelfGet).Validate() },
+		},
+		{
+			name: "self.update",
+			in:   &protocol.SelfUpdate{V: protocol.Version, Type: protocol.TypeSelfUpdate, RID: "12", Name: "门口", DefaultDelayMs: ptrInt64(10000)},
+			typ:  protocol.TypeSelfUpdate,
+			val:  func(v any) error { return v.(*protocol.SelfUpdate).Validate() },
+		},
+		{
+			name: "self.talk_password",
+			in:   &protocol.SelfTalkPassword{V: protocol.Version, Type: protocol.TypeSelfTalkPassword, RID: "13", TalkPassword: ""},
+			typ:  protocol.TypeSelfTalkPassword,
+			val:  func(v any) error { return v.(*protocol.SelfTalkPassword).Validate() },
+		},
+		{
+			name: "self.login_password",
+			in: &protocol.SelfLoginPassword{
+				V: protocol.Version, Type: protocol.TypeSelfLoginPassword, RID: "14",
+				OldPassword: "oldpass12", NewPassword: "newpass12",
+			},
+			typ: protocol.TypeSelfLoginPassword,
+			val: func(v any) error { return v.(*protocol.SelfLoginPassword).Validate() },
+		},
+		{
+			name: "self.logout",
+			in:   &protocol.SelfLogout{V: protocol.Version, Type: protocol.TypeSelfLogout, RID: "24"},
+			typ:  protocol.TypeSelfLogout,
+			val:  func(v any) error { return v.(*protocol.SelfLogout).Validate() },
+		},
+		{
+			name: "group.create",
+			in: &protocol.GroupCreate{
+				V: protocol.Version, Type: protocol.TypeGroupCreate, RID: "15", ID: "",
+				Name: "一组", Members: []protocol.GroupMemberIn{{ID: "b", TalkPassword: "secret"}},
+			},
+			typ: protocol.TypeGroupCreate,
+			val: func(v any) error { return v.(*protocol.GroupCreate).Validate() },
+		},
+		{
+			name: "group.add",
+			in: &protocol.GroupAdd{
+				V: protocol.Version, Type: protocol.TypeGroupAdd, RID: "16", GroupID: "g_ab12cd34",
+				Members: []protocol.GroupMemberIn{{ID: "c"}},
+			},
+			typ: protocol.TypeGroupAdd,
+			val: func(v any) error { return v.(*protocol.GroupAdd).Validate() },
+		},
+		{
+			name: "group.remove",
+			in:   &protocol.GroupRemove{V: protocol.Version, Type: protocol.TypeGroupRemove, RID: "17", GroupID: "g_ab12cd34", EndpointID: "c"},
+			typ:  protocol.TypeGroupRemove,
+			val:  func(v any) error { return v.(*protocol.GroupRemove).Validate() },
+		},
+		{
+			name: "group.leave",
+			in:   &protocol.GroupLeave{V: protocol.Version, Type: protocol.TypeGroupLeave, RID: "18", GroupID: "g_ab12cd34"},
+			typ:  protocol.TypeGroupLeave,
+			val:  func(v any) error { return v.(*protocol.GroupLeave).Validate() },
+		},
+		{
+			name: "group.transfer",
+			in:   &protocol.GroupTransfer{V: protocol.Version, Type: protocol.TypeGroupTransfer, RID: "19", GroupID: "g_ab12cd34", EndpointID: "b"},
+			typ:  protocol.TypeGroupTransfer,
+			val:  func(v any) error { return v.(*protocol.GroupTransfer).Validate() },
+		},
+		{
+			name: "group.rename",
+			in:   &protocol.GroupRename{V: protocol.Version, Type: protocol.TypeGroupRename, RID: "20", GroupID: "g_ab12cd34", Name: "新名"},
+			typ:  protocol.TypeGroupRename,
+			val:  func(v any) error { return v.(*protocol.GroupRename).Validate() },
+		},
+		{
+			name: "group.dissolve",
+			in:   &protocol.GroupDissolve{V: protocol.Version, Type: protocol.TypeGroupDissolve, RID: "21", GroupID: "g_ab12cd34"},
+			typ:  protocol.TypeGroupDissolve,
+			val:  func(v any) error { return v.(*protocol.GroupDissolve).Validate() },
+		},
+		{
+			name: "group.list",
+			in:   &protocol.GroupList{V: protocol.Version, Type: protocol.TypeGroupList, RID: "22", Limit: 100},
+			typ:  protocol.TypeGroupList,
+			val:  func(v any) error { return v.(*protocol.GroupList).Validate() },
+		},
+		{
+			name: "group.get",
+			in:   &protocol.GroupGet{V: protocol.Version, Type: protocol.TypeGroupGet, RID: "23", GroupID: "g_ab12cd34", Limit: 100},
+			typ:  protocol.TypeGroupGet,
+			val:  func(v any) error { return v.(*protocol.GroupGet).Validate() },
+		},
+		{
+			name: "group_event",
+			in: &protocol.GroupEvent{
+				V: protocol.Version, Type: protocol.TypeGroupEvent, GroupID: "g_ab12cd34",
+				Event: "member_added", EndpointID: "c", AtMs: 1,
+			},
+			typ: protocol.TypeGroupEvent,
+			val: func(v any) error { return v.(*protocol.GroupEvent).Validate() },
+		},
+		{
+			name: "fatal",
+			in:   &protocol.Fatal{V: protocol.Version, Type: protocol.TypeFatal, Reason: "disabled"},
+			typ:  protocol.TypeFatal,
+			val:  func(v any) error { return v.(*protocol.Fatal).Validate() },
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			raw := mustMarshal(t, tc.in)
+			got, err := protocol.Decode(raw)
+			if err != nil {
+				t.Fatalf("Decode: %v", err)
+			}
+			back := mustMarshal(t, got)
+			var a, b any
+			if err := json.Unmarshal(raw, &a); err != nil {
+				t.Fatal(err)
+			}
+			if err := json.Unmarshal(back, &b); err != nil {
+				t.Fatal(err)
+			}
+			aj, _ := json.Marshal(a)
+			bj, _ := json.Marshal(b)
+			if !bytes.Equal(aj, bj) {
+				t.Fatalf("round-trip mismatch\n%s\n%s", aj, bj)
+			}
+			peek := struct {
+				Type string `json:"type"`
+			}{}
+			_ = json.Unmarshal(raw, &peek)
+			if peek.Type != tc.typ {
+				t.Fatalf("type=%s want %s", peek.Type, tc.typ)
+			}
+			if err := tc.val(got); err != nil {
+				t.Fatalf("Validate: %v", err)
+			}
+		})
+	}
+}
+
+func TestRegisterRoundTrip(t *testing.T) {
+	req := &protocol.RegisterRequest{
+		RegistrationCode: "code",
+		ID:               "device-1",
+		LoginPassword:    "password1",
+		Name:             "门口",
+		TalkPassword:     "talk",
+	}
+	raw := mustMarshal(t, req)
+	got, err := protocol.DecodeRegister(raw)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := got.Validate(); err != nil {
+		t.Fatal(err)
+	}
+	resp := protocol.RegisterResponse{OK: true, Data: protocol.RegisterData{ID: "e_ab12cd34", LoginPassword: "generated"}}
+	rb := mustMarshal(t, resp)
+	var decoded protocol.RegisterResponse
+	if err := protocol.Unmarshal(rb, &decoded); err != nil {
+		t.Fatal(err)
+	}
+	if !decoded.OK || decoded.Data.ID != "e_ab12cd34" {
+		t.Fatalf("unexpected resp: %+v", decoded)
+	}
+}
+
+func TestValidateTable(t *testing.T) {
+	lim := protocol.DefaultLimits()
+	cases := []struct {
+		name     string
+		check    func() error
+		wantCode string
+	}{
+		{
+			name: "endpoint_id_uppercase",
+			check: func() error {
+				return (&protocol.Send{
+					V: protocol.Version, Type: protocol.TypeSend, RID: "1", ID: "m1",
+					To:   protocol.Target{Kind: protocol.TargetEndpoint, ID: "Device"},
+					Body: protocol.Body{Enc: protocol.EncUTF8, Data: "x"},
+				}).Validate(lim)
+			},
+			wantCode: protocol.CodeBadRequest,
+		},
+		{
+			name: "message_id_allows_upper",
+			check: func() error {
+				return (&protocol.Send{
+					V: protocol.Version, Type: protocol.TypeSend, RID: "1", ID: "Msg_1.A-b",
+					To:   protocol.Target{Kind: protocol.TargetEndpoint, ID: "device-1"},
+					Body: protocol.Body{Enc: protocol.EncUTF8, Data: "x"},
+				}).Validate(lim)
+			},
+		},
+		{
+			name: "send_at_and_delay_mutex",
+			check: func() error {
+				return (&protocol.Send{
+					V: protocol.Version, Type: protocol.TypeSend, RID: "1", ID: "m1",
+					To:       protocol.Target{Kind: protocol.TargetEndpoint, ID: "device-1"},
+					Body:     protocol.Body{Enc: protocol.EncUTF8, Data: "x"},
+					SendAtMs: ptrInt64(1), DelayMs: ptrInt64(2),
+				}).Validate(lim)
+			},
+			wantCode: protocol.CodeBadRequest,
+		},
+		{
+			name: "body_too_large",
+			check: func() error {
+				return (&protocol.Send{
+					V: protocol.Version, Type: protocol.TypeSend, RID: "1", ID: "m1",
+					To:   protocol.Target{Kind: protocol.TargetEndpoint, ID: "device-1"},
+					Body: protocol.Body{Enc: protocol.EncUTF8, Data: strings.Repeat("a", 10)},
+				}).Validate(protocol.Limits{MaxBodyBytes: 5, MaxMetaBytes: 4096, MaxFrameBytes: 786432})
+			},
+			wantCode: protocol.CodeBodyTooLarge,
+		},
+		{
+			name: "meta_too_large",
+			check: func() error {
+				return (&protocol.Send{
+					V: protocol.Version, Type: protocol.TypeSend, RID: "1", ID: "m1",
+					To:   protocol.Target{Kind: protocol.TargetEndpoint, ID: "device-1"},
+					Body: protocol.Body{Enc: protocol.EncUTF8, Data: "x"},
+					Meta: map[string]any{"k": strings.Repeat("v", 100)},
+				}).Validate(protocol.Limits{MaxBodyBytes: 262144, MaxMetaBytes: 20, MaxFrameBytes: 786432})
+			},
+			wantCode: protocol.CodeMetaTooLarge,
+		},
+		{
+			name: "login_password_nst_prefix",
+			check: func() error {
+				return (&protocol.RegisterRequest{LoginPassword: "nst_notallowed"}).Validate()
+			},
+			wantCode: protocol.CodeBadRequest,
+		},
+		{
+			name: "self_login_password_nst_prefix",
+			check: func() error {
+				return (&protocol.SelfLoginPassword{
+					V: protocol.Version, Type: protocol.TypeSelfLoginPassword, RID: "1",
+					OldPassword: "oldpass12", NewPassword: "nst_tokenlike",
+				}).Validate()
+			},
+			wantCode: protocol.CodeBadRequest,
+		},
+		{
+			name: "hello_max_receive_too_small",
+			check: func() error {
+				return (&protocol.Hello{
+					V: protocol.Version, Type: protocol.TypeHello, RID: "1",
+					MaxReceiveBytes: ptrInt(100),
+				}).Validate()
+			},
+			wantCode: protocol.CodeBadRequest,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := tc.check()
+			if tc.wantCode == "" {
+				if err != nil {
+					t.Fatalf("unexpected err: %v", err)
+				}
+				return
+			}
+			pe, ok := err.(*protocol.Error)
+			if !ok {
+				t.Fatalf("want *protocol.Error, got %T %v", err, err)
+			}
+			if pe.Code != tc.wantCode {
+				t.Fatalf("code=%s want %s", pe.Code, tc.wantCode)
+			}
+		})
+	}
+}
+
+func TestRequestFingerprintMetaKeyOrderInsensitive(t *testing.T) {
+	base := func(meta map[string]any) *protocol.Send {
+		return &protocol.Send{
+			V: protocol.Version, Type: protocol.TypeSend, RID: "rid-ignored", ID: "m1",
+			To:           protocol.Target{Kind: protocol.TargetEndpoint, ID: "device-1"},
+			Body:         protocol.Body{Enc: protocol.EncUTF8, ContentType: "text/plain", Data: "hello"},
+			Meta:         meta,
+			Offline:      &protocol.OfflineOpts{Keep: true, TTLSeconds: ptrInt64(60)},
+			Receipt:      ptrBool(true),
+			TalkPassword: "should-not-affect",
+		}
+	}
+	fp1, err := protocol.RequestFingerprint(base(map[string]any{"a": 1, "b": "x", "c": true}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	fp2, err := protocol.RequestFingerprint(base(map[string]any{"c": true, "b": "x", "a": 1}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fp1 != fp2 {
+		t.Fatalf("fingerprint depends on meta key order: %s vs %s", fp1, fp2)
+	}
+	// rid / talk_password 变化不应影响
+	s3 := base(map[string]any{"a": 1, "b": "x", "c": true})
+	s3.RID = "other-rid"
+	s3.TalkPassword = "other"
+	fp3, err := protocol.RequestFingerprint(s3)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fp3 != fp1 {
+		t.Fatalf("rid/talk_password affected fingerprint")
+	}
+	// 正文变化应影响
+	s4 := base(map[string]any{"a": 1, "b": "x", "c": true})
+	s4.Body.Data = "hello!"
+	fp4, err := protocol.RequestFingerprint(s4)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fp4 == fp1 {
+		t.Fatalf("body change should change fingerprint")
+	}
+}
+
+func TestErrorCodesDefined(t *testing.T) {
+	codes := []string{
+		protocol.CodeBadRequest, protocol.CodeNotReady, protocol.CodeUnauthorized,
+		protocol.CodeForbidden, protocol.CodeNotFound, protocol.CodeInvalidTarget,
+		protocol.CodeConflict, protocol.CodeIDTaken, protocol.CodeBodyTooLarge,
+		protocol.CodeMetaTooLarge, protocol.CodeFrameTooLarge, protocol.CodeResponseTooLarge,
+		protocol.CodeTalkPasswordRequired, protocol.CodeTalkPasswordInvalid,
+		protocol.CodeRateLimited, protocol.CodeNotMember, protocol.CodeOwnerCannotLeave,
+		protocol.CodeGroupFull, protocol.CodeQuotaExceeded, protocol.CodeEndpointDisabled,
+		protocol.CodeRegistrationClosed, protocol.CodeRegistrationCodeInvalid, protocol.CodeBusy,
+	}
+	if len(codes) != 23 {
+		t.Fatalf("want 23 error codes, got %d", len(codes))
+	}
+	seen := map[string]bool{}
+	for _, c := range codes {
+		if c == "" || seen[c] {
+			t.Fatalf("bad code %q", c)
+		}
+		seen[c] = true
+	}
+}
diff --git a/internal/protocol/types.go b/internal/protocol/types.go
new file mode 100644
index 0000000..66d6f7c
--- /dev/null
+++ b/internal/protocol/types.go
@@ -0,0 +1,393 @@
+package protocol
+
+import "encoding/json"
+
+// Target 是发送目标。
+type Target struct {
+	Kind string `json:"kind"`
+	ID   string `json:"id"`
+}
+
+// Body 是消息正文。
+type Body struct {
+	Enc         string `json:"enc"`
+	ContentType string `json:"content_type,omitempty"`
+	Data        string `json:"data"`
+}
+
+// OfflineOpts 是离线保留选项。
+type OfflineOpts struct {
+	Keep       bool   `json:"keep"`
+	TTLSeconds *int64 `json:"ttl_seconds,omitempty"`
+}
+
+// Limits 是与服务器配置相关的校验上限。
+type Limits struct {
+	MaxBodyBytes  int
+	MaxMetaBytes  int
+	MaxFrameBytes int
+}
+
+// DefaultLimits 返回 DEVELOPMENT 示例中的默认上限。
+func DefaultLimits() Limits {
+	return Limits{
+		MaxBodyBytes:  DefaultMaxBodyBytes,
+		MaxMetaBytes:  DefaultMaxMetaBytes,
+		MaxFrameBytes: DefaultMaxFrameBytes,
+	}
+}
+
+func (l Limits) withDefaults() Limits {
+	if l.MaxBodyBytes <= 0 {
+		l.MaxBodyBytes = DefaultMaxBodyBytes
+	}
+	if l.MaxMetaBytes <= 0 {
+		l.MaxMetaBytes = DefaultMaxMetaBytes
+	}
+	if l.MaxFrameBytes <= 0 {
+		l.MaxFrameBytes = DefaultMaxFrameBytes
+	}
+	return l
+}
+
+// Hello 是握手请求（第 6.1 节）。
+type Hello struct {
+	V               int    `json:"v"`
+	Type            string `json:"type"`
+	RID             string `json:"rid"`
+	MaxReceiveBytes *int   `json:"max_receive_bytes,omitempty"`
+	Client          string `json:"client,omitempty"`
+}
+
+// HelloData 是握手成功响应 data。
+type HelloData struct {
+	ServerTimeMs       int64  `json:"server_time_ms"`
+	ServerVersion      string `json:"server_version"`
+	MaxBodyBytes       int    `json:"max_body_bytes"`
+	MaxMetaBytes       int    `json:"max_meta_bytes"`
+	MaxFrameBytes      int    `json:"max_frame_bytes"`
+	MaxTTLSeconds      int64  `json:"max_ttl_seconds"`
+	MaxScheduleSeconds int64  `json:"max_schedule_seconds"`
+	AckTimeoutSeconds  int64  `json:"ack_timeout_seconds"`
+	SessionToken       string `json:"session_token,omitempty"`
+}
+
+// Resp 是通用响应帧。
+type Resp struct {
+	V     int             `json:"v"`
+	Type  string          `json:"type"`
+	RID   string          `json:"rid"`
+	OK    bool            `json:"ok"`
+	Data  json.RawMessage `json:"data,omitempty"`
+	Error *ErrorBody      `json:"error,omitempty"`
+}
+
+// Send 是发送请求（第 6.2 节）。
+type Send struct {
+	V            int            `json:"v"`
+	Type         string         `json:"type"`
+	RID          string         `json:"rid"`
+	ID           string         `json:"id"`
+	To           Target         `json:"to"`
+	Body         Body           `json:"body"`
+	Meta         map[string]any `json:"meta,omitempty"`
+	DelayMs      *int64         `json:"delay_ms,omitempty"`
+	SendAtMs     *int64         `json:"send_at_ms,omitempty"`
+	Offline      *OfflineOpts   `json:"offline,omitempty"`
+	Receipt      *bool          `json:"receipt,omitempty"`
+	TalkPassword string         `json:"talk_password,omitempty"`
+}
+
+// SendData 是发送成功响应 data。
+type SendData struct {
+	ID       string `json:"id"`
+	SendAtMs int64  `json:"send_at_ms"`
+	State    string `json:"state"`
+}
+
+// Msg 是下行消息（第 6.3 节）。
+type Msg struct {
+	V        int            `json:"v"`
+	Type     string         `json:"type"`
+	ID       string         `json:"id"`
+	From     string         `json:"from"`
+	To       Target         `json:"to"`
+	Body     Body           `json:"body"`
+	Meta     map[string]any `json:"meta,omitempty"`
+	SendAtMs int64          `json:"send_at_ms"`
+}
+
+// Ack 是确认请求。
+type Ack struct {
+	V    int    `json:"v"`
+	Type string `json:"type"`
+	RID  string `json:"rid"`
+	From string `json:"from"`
+	ID   string `json:"id"`
+}
+
+// Recall 是撤回请求。
+type Recall struct {
+	V    int    `json:"v"`
+	Type string `json:"type"`
+	RID  string `json:"rid"`
+	ID   string `json:"id"`
+}
+
+// RecallData 是撤回响应 data。
+type RecallData struct {
+	Result   string `json:"result"`
+	Recalled int    `json:"recalled"`
+	Accepted int    `json:"accepted"`
+	Other    int    `json:"other"`
+}
+
+// Status 是状态查询请求。
+type Status struct {
+	V      int    `json:"v"`
+	Type   string `json:"type"`
+	RID    string `json:"rid"`
+	ID     string `json:"id"`
+	Cursor string `json:"cursor,omitempty"`
+	Limit  int    `json:"limit,omitempty"`
+}
+
+// Receipt 是回执下行。
+type Receipt struct {
+	V          int    `json:"v"`
+	Type       string `json:"type"`
+	ReceiptID  string `json:"receipt_id"`
+	ID         string `json:"id"`
+	EndpointID string `json:"endpoint_id"`
+	State      string `json:"state"`
+	Reason     string `json:"reason"`
+	AtMs       int64  `json:"at_ms"`
+}
+
+// ReceiptAck 是回执确认。
+type ReceiptAck struct {
+	V         int    `json:"v"`
+	Type      string `json:"type"`
+	RID       string `json:"rid"`
+	ReceiptID string `json:"receipt_id"`
+}
+
+// Revoked 是已推送消息作废通知。
+type Revoked struct {
+	V      int    `json:"v"`
+	Type   string `json:"type"`
+	ID     string `json:"id"`
+	From   string `json:"from"`
+	Reason string `json:"reason"`
+}
+
+// PresenceGet 查询在线状态。
+type PresenceGet struct {
+	V    int      `json:"v"`
+	Type string   `json:"type"`
+	RID  string   `json:"rid"`
+	IDs  []string `json:"ids"`
+}
+
+// DirectoryList 列目录。
+type DirectoryList struct {
+	V      int    `json:"v"`
+	Type   string `json:"type"`
+	RID    string `json:"rid"`
+	Cursor string `json:"cursor,omitempty"`
+	Limit  int    `json:"limit,omitempty"`
+	Query  string `json:"query,omitempty"`
+}
+
+// PresenceWatch 订阅上下线。
+type PresenceWatch struct {
+	V    int      `json:"v"`
+	Type string   `json:"type"`
+	RID  string   `json:"rid"`
+	IDs  []string `json:"ids,omitempty"`
+	All  bool     `json:"all,omitempty"`
+}
+
+// Presence 是上下线通知。
+type Presence struct {
+	V      int    `json:"v"`
+	Type   string `json:"type"`
+	ID     string `json:"id"`
+	Online bool   `json:"online"`
+	AtMs   int64  `json:"at_ms"`
+}
+
+// Unlock 解锁对话密码。
+type Unlock struct {
+	V            int    `json:"v"`
+	Type         string `json:"type"`
+	RID          string `json:"rid"`
+	EndpointID   string `json:"endpoint_id"`
+	TalkPassword string `json:"talk_password"`
+}
+
+// SelfGet 读取自己的资料。
+type SelfGet struct {
+	V    int    `json:"v"`
+	Type string `json:"type"`
+	RID  string `json:"rid"`
+}
+
+// SelfUpdate 更新自己的资料。
+type SelfUpdate struct {
+	V              int    `json:"v"`
+	Type           string `json:"type"`
+	RID            string `json:"rid"`
+	Name           string `json:"name,omitempty"`
+	DefaultDelayMs *int64 `json:"default_delay_ms,omitempty"`
+}
+
+// SelfTalkPassword 设置对话密码。
+type SelfTalkPassword struct {
+	V            int    `json:"v"`
+	Type         string `json:"type"`
+	RID          string `json:"rid"`
+	TalkPassword string `json:"talk_password"`
+}
+
+// SelfLoginPassword 修改登录密码。
+type SelfLoginPassword struct {
+	V           int    `json:"v"`
+	Type        string `json:"type"`
+	RID         string `json:"rid"`
+	OldPassword string `json:"old_password"`
+	NewPassword string `json:"new_password"`
+}
+
+// SelfLogout 退出登录并作废会话令牌。
+type SelfLogout struct {
+	V    int    `json:"v"`
+	Type string `json:"type"`
+	RID  string `json:"rid"`
+}
+
+// GroupMemberIn 是建群/加群时的成员项。
+type GroupMemberIn struct {
+	ID           string `json:"id"`
+	TalkPassword string `json:"talk_password,omitempty"`
+}
+
+// GroupCreate 建群。
+type GroupCreate struct {
+	V       int             `json:"v"`
+	Type    string          `json:"type"`
+	RID     string          `json:"rid"`
+	ID      string          `json:"id"`
+	Name    string          `json:"name"`
+	Members []GroupMemberIn `json:"members"`
+}
+
+// GroupAdd 加成员。
+type GroupAdd struct {
+	V       int             `json:"v"`
+	Type    string          `json:"type"`
+	RID     string          `json:"rid"`
+	GroupID string          `json:"group_id"`
+	Members []GroupMemberIn `json:"members"`
+}
+
+// GroupRemove 移除成员。
+type GroupRemove struct {
+	V          int    `json:"v"`
+	Type       string `json:"type"`
+	RID        string `json:"rid"`
+	GroupID    string `json:"group_id"`
+	EndpointID string `json:"endpoint_id"`
+}
+
+// GroupLeave 退群。
+type GroupLeave struct {
+	V       int    `json:"v"`
+	Type    string `json:"type"`
+	RID     string `json:"rid"`
+	GroupID string `json:"group_id"`
+}
+
+// GroupTransfer 转让群主。
+type GroupTransfer struct {
+	V          int    `json:"v"`
+	Type       string `json:"type"`
+	RID        string `json:"rid"`
+	GroupID    string `json:"group_id"`
+	EndpointID string `json:"endpoint_id"`
+}
+
+// GroupRename 改群名。
+type GroupRename struct {
+	V       int    `json:"v"`
+	Type    string `json:"type"`
+	RID     string `json:"rid"`
+	GroupID string `json:"group_id"`
+	Name    string `json:"name"`
+}
+
+// GroupDissolve 解散群。
+type GroupDissolve struct {
+	V       int    `json:"v"`
+	Type    string `json:"type"`
+	RID     string `json:"rid"`
+	GroupID string `json:"group_id"`
+}
+
+// GroupList 列出我的群。
+type GroupList struct {
+	V      int    `json:"v"`
+	Type   string `json:"type"`
+	RID    string `json:"rid"`
+	Cursor string `json:"cursor,omitempty"`
+	Limit  int    `json:"limit,omitempty"`
+}
+
+// GroupGet 获取群详情。
+type GroupGet struct {
+	V       int    `json:"v"`
+	Type    string `json:"type"`
+	RID     string `json:"rid"`
+	GroupID string `json:"group_id"`
+	Cursor  string `json:"cursor,omitempty"`
+	Limit   int    `json:"limit,omitempty"`
+}
+
+// GroupEvent 是群事件下行。
+type GroupEvent struct {
+	V          int    `json:"v"`
+	Type       string `json:"type"`
+	GroupID    string `json:"group_id"`
+	Event      string `json:"event"`
+	EndpointID string `json:"endpoint_id,omitempty"`
+	AtMs       int64  `json:"at_ms"`
+}
+
+// Fatal 是致命错误下行。
+type Fatal struct {
+	V      int    `json:"v"`
+	Type   string `json:"type"`
+	Reason string `json:"reason"`
+}
+
+// RegisterRequest 是 HTTP 注册请求（第 6.9 节）。
+type RegisterRequest struct {
+	RegistrationCode string `json:"registration_code"`
+	ID               string `json:"id"`
+	LoginPassword    string `json:"login_password"`
+	Name             string `json:"name,omitempty"`
+	TalkPassword     string `json:"talk_password,omitempty"`
+}
+
+// RegisterData 是注册成功 data。
+type RegisterData struct {
+	ID            string `json:"id"`
+	LoginPassword string `json:"login_password,omitempty"`
+}
+
+// RegisterResponse 是注册 HTTP 响应。
+type RegisterResponse struct {
+	OK    bool         `json:"ok"`
+	Data  RegisterData `json:"data,omitempty"`
+	Error *ErrorBody   `json:"error,omitempty"`
+}
diff --git a/internal/protocol/validate.go b/internal/protocol/validate.go
new file mode 100644
index 0000000..9d9785b
--- /dev/null
+++ b/internal/protocol/validate.go
@@ -0,0 +1,750 @@
+package protocol
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"unicode/utf8"
+)
+
+// DecodeBody 按 enc 解码正文，返回原始字节。
+func DecodeBody(b Body) ([]byte, error) {
+	switch b.Enc {
+	case EncUTF8:
+		if !utf8.ValidString(b.Data) {
+			return nil, badRequest("body data is not valid utf8")
+		}
+		return []byte(b.Data), nil
+	case EncBase64:
+		raw, err := base64.StdEncoding.DecodeString(b.Data)
+		if err != nil {
+			return nil, badRequest("body data is not valid base64")
+		}
+		return raw, nil
+	default:
+		return nil, badRequest("invalid body.enc")
+	}
+}
+
+// EffectiveContentType 返回带默认值的 content_type。
+func EffectiveContentType(b Body) string {
+	if b.ContentType != "" {
+		return b.ContentType
+	}
+	switch b.Enc {
+	case EncBase64:
+		return DefaultContentTypeBase64
+	default:
+		return DefaultContentTypeUTF8
+	}
+}
+
+// EffectiveReceipt 返回 receipt 有效值（默认 true）。
+func EffectiveReceipt(s *Send) bool {
+	if s.Receipt == nil {
+		return true
+	}
+	return *s.Receipt
+}
+
+// EffectiveOfflineKeep 返回 offline.keep（默认 false）。
+func EffectiveOfflineKeep(s *Send) bool {
+	if s.Offline == nil {
+		return false
+	}
+	return s.Offline.Keep
+}
+
+// EffectiveOfflineTTL 返回 offline.ttl_seconds 有效值；keep 为 false 时为 0。
+func EffectiveOfflineTTL(s *Send) int64 {
+	if !EffectiveOfflineKeep(s) {
+		return 0
+	}
+	if s.Offline.TTLSeconds == nil {
+		return DefaultOfflineTTL
+	}
+	return *s.Offline.TTLSeconds
+}
+
+func checkMetaSize(meta map[string]any, maxBytes int) error {
+	if meta == nil {
+		return nil
+	}
+	b, err := Marshal(meta)
+	if err != nil {
+		return badRequest("invalid meta")
+	}
+	if len(b) > maxBytes {
+		return metaTooLarge("meta exceeds limit")
+	}
+	return nil
+}
+
+func checkBodySize(b Body, maxBytes int) ([]byte, error) {
+	raw, err := DecodeBody(b)
+	if err != nil {
+		if pe, ok := err.(*Error); ok {
+			return nil, pe
+		}
+		return nil, badRequest(err.Error())
+	}
+	if len(raw) > maxBytes {
+		return nil, bodyTooLarge("body exceeds limit")
+	}
+	if b.ContentType != "" && utf8.RuneCountInString(b.ContentType) > MaxContentTypeLen {
+		return nil, badRequest("content_type too long")
+	}
+	return raw, nil
+}
+
+func checkPageLimit(limit int) error {
+	if limit < 0 || limit > MaxPageLimit {
+		return badRequest("limit out of range")
+	}
+	return nil
+}
+
+// Validate 校验 Hello。
+func (h *Hello) Validate() error {
+	if err := requireVersion(h.V); err != nil {
+		return err
+	}
+	if err := requireType(h.Type, TypeHello); err != nil {
+		return err
+	}
+	if err := requireRID(h.RID); err != nil {
+		return err
+	}
+	if h.MaxReceiveBytes != nil && *h.MaxReceiveBytes < MinMaxReceiveBytes {
+		return badRequest("max_receive_bytes too small")
+	}
+	return nil
+}
+
+// Validate 校验 Resp。
+func (r *Resp) Validate() error {
+	if err := requireVersion(r.V); err != nil {
+		return err
+	}
+	if err := requireType(r.Type, TypeResp); err != nil {
+		return err
+	}
+	if err := requireRID(r.RID); err != nil {
+		return err
+	}
+	if !r.OK && (r.Error == nil || r.Error.Code == "") {
+		return badRequest("error response missing code")
+	}
+	return nil
+}
+
+// Validate 校验 Send（含正文/meta 大小与 send_at_ms/delay_ms 互斥）。
+func (s *Send) Validate(lim Limits) error {
+	lim = lim.withDefaults()
+	if err := requireVersion(s.V); err != nil {
+		return err
+	}
+	if err := requireType(s.Type, TypeSend); err != nil {
+		return err
+	}
+	if err := requireRID(s.RID); err != nil {
+		return err
+	}
+	if !ValidMessageID(s.ID) {
+		return badRequest("invalid message id")
+	}
+	if s.To.Kind != TargetEndpoint && s.To.Kind != TargetGroup {
+		return badRequest("invalid to.kind")
+	}
+	if !ValidEndpointID(s.To.ID) {
+		return badRequest("invalid to.id")
+	}
+	if _, err := checkBodySize(s.Body, lim.MaxBodyBytes); err != nil {
+		return err
+	}
+	if err := checkMetaSize(s.Meta, lim.MaxMetaBytes); err != nil {
+		return err
+	}
+	if s.SendAtMs != nil && s.DelayMs != nil {
+		return badRequest("send_at_ms and delay_ms are mutually exclusive")
+	}
+	n, err := FrameBytes(s)
+	if err != nil {
+		return badRequest("cannot encode frame")
+	}
+	if n > lim.MaxFrameBytes {
+		return frameTooLarge("frame exceeds limit")
+	}
+	return nil
+}
+
+// Validate 校验 Msg。
+func (m *Msg) Validate(lim Limits) error {
+	lim = lim.withDefaults()
+	if err := requireVersion(m.V); err != nil {
+		return err
+	}
+	if err := requireType(m.Type, TypeMsg); err != nil {
+		return err
+	}
+	if !ValidMessageID(m.ID) {
+		return badRequest("invalid message id")
+	}
+	if !ValidEndpointID(m.From) {
+		return badRequest("invalid from")
+	}
+	if m.To.Kind != TargetEndpoint && m.To.Kind != TargetGroup {
+		return badRequest("invalid to.kind")
+	}
+	if !ValidEndpointID(m.To.ID) {
+		return badRequest("invalid to.id")
+	}
+	if _, err := checkBodySize(m.Body, lim.MaxBodyBytes); err != nil {
+		return err
+	}
+	if err := checkMetaSize(m.Meta, lim.MaxMetaBytes); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Validate 校验 Ack。
+func (a *Ack) Validate() error {
+	if err := requireVersion(a.V); err != nil {
+		return err
+	}
+	if err := requireType(a.Type, TypeAck); err != nil {
+		return err
+	}
+	if err := requireRID(a.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(a.From) {
+		return badRequest("invalid from")
+	}
+	if !ValidMessageID(a.ID) {
+		return badRequest("invalid message id")
+	}
+	return nil
+}
+
+// Validate 校验 Recall。
+func (r *Recall) Validate() error {
+	if err := requireVersion(r.V); err != nil {
+		return err
+	}
+	if err := requireType(r.Type, TypeRecall); err != nil {
+		return err
+	}
+	if err := requireRID(r.RID); err != nil {
+		return err
+	}
+	if !ValidMessageID(r.ID) {
+		return badRequest("invalid message id")
+	}
+	return nil
+}
+
+// Validate 校验 Status。
+func (s *Status) Validate() error {
+	if err := requireVersion(s.V); err != nil {
+		return err
+	}
+	if err := requireType(s.Type, TypeStatus); err != nil {
+		return err
+	}
+	if err := requireRID(s.RID); err != nil {
+		return err
+	}
+	if !ValidMessageID(s.ID) {
+		return badRequest("invalid message id")
+	}
+	if err := checkPageLimit(s.Limit); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Validate 校验 Receipt。
+func (r *Receipt) Validate() error {
+	if err := requireVersion(r.V); err != nil {
+		return err
+	}
+	if err := requireType(r.Type, TypeReceipt); err != nil {
+		return err
+	}
+	if r.ReceiptID == "" {
+		return badRequest("missing receipt_id")
+	}
+	if !ValidMessageID(r.ID) {
+		return badRequest("invalid message id")
+	}
+	if r.EndpointID != "" && !ValidEndpointID(r.EndpointID) {
+		return badRequest("invalid endpoint_id")
+	}
+	return nil
+}
+
+// Validate 校验 ReceiptAck。
+func (r *ReceiptAck) Validate() error {
+	if err := requireVersion(r.V); err != nil {
+		return err
+	}
+	if err := requireType(r.Type, TypeReceiptAck); err != nil {
+		return err
+	}
+	if err := requireRID(r.RID); err != nil {
+		return err
+	}
+	if r.ReceiptID == "" {
+		return badRequest("missing receipt_id")
+	}
+	return nil
+}
+
+// Validate 校验 Revoked。
+func (r *Revoked) Validate() error {
+	if err := requireVersion(r.V); err != nil {
+		return err
+	}
+	if err := requireType(r.Type, TypeRevoked); err != nil {
+		return err
+	}
+	if !ValidMessageID(r.ID) {
+		return badRequest("invalid message id")
+	}
+	if !ValidEndpointID(r.From) {
+		return badRequest("invalid from")
+	}
+	if r.Reason == "" {
+		return badRequest("missing reason")
+	}
+	return nil
+}
+
+// Validate 校验 PresenceGet。
+func (p *PresenceGet) Validate() error {
+	if err := requireVersion(p.V); err != nil {
+		return err
+	}
+	if err := requireType(p.Type, TypePresenceGet); err != nil {
+		return err
+	}
+	if err := requireRID(p.RID); err != nil {
+		return err
+	}
+	if len(p.IDs) > MaxPresenceGetIDs {
+		return badRequest("ids count out of range")
+	}
+	for _, id := range p.IDs {
+		if !ValidEndpointID(id) {
+			return badRequest("invalid id in ids")
+		}
+	}
+	return nil
+}
+
+// Validate 校验 DirectoryList。
+func (d *DirectoryList) Validate() error {
+	if err := requireVersion(d.V); err != nil {
+		return err
+	}
+	if err := requireType(d.Type, TypeDirectoryList); err != nil {
+		return err
+	}
+	if err := requireRID(d.RID); err != nil {
+		return err
+	}
+	if err := checkPageLimit(d.Limit); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Validate 校验 PresenceWatch。
+func (p *PresenceWatch) Validate() error {
+	if err := requireVersion(p.V); err != nil {
+		return err
+	}
+	if err := requireType(p.Type, TypePresenceWatch); err != nil {
+		return err
+	}
+	if err := requireRID(p.RID); err != nil {
+		return err
+	}
+	if len(p.IDs) > MaxPresenceWatchIDs {
+		return badRequest("ids count out of range")
+	}
+	for _, id := range p.IDs {
+		if !ValidEndpointID(id) {
+			return badRequest("invalid id in ids")
+		}
+	}
+	return nil
+}
+
+// Validate 校验 Presence。
+func (p *Presence) Validate() error {
+	if err := requireVersion(p.V); err != nil {
+		return err
+	}
+	if err := requireType(p.Type, TypePresence); err != nil {
+		return err
+	}
+	if !ValidEndpointID(p.ID) {
+		return badRequest("invalid id")
+	}
+	return nil
+}
+
+// Validate 校验 Unlock。
+func (u *Unlock) Validate() error {
+	if err := requireVersion(u.V); err != nil {
+		return err
+	}
+	if err := requireType(u.Type, TypeUnlock); err != nil {
+		return err
+	}
+	if err := requireRID(u.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(u.EndpointID) {
+		return badRequest("invalid endpoint_id")
+	}
+	return nil
+}
+
+// Validate 校验 SelfGet。
+func (s *SelfGet) Validate() error {
+	if err := requireVersion(s.V); err != nil {
+		return err
+	}
+	if err := requireType(s.Type, TypeSelfGet); err != nil {
+		return err
+	}
+	return requireRID(s.RID)
+}
+
+// Validate 校验 SelfUpdate。
+func (s *SelfUpdate) Validate() error {
+	if err := requireVersion(s.V); err != nil {
+		return err
+	}
+	if err := requireType(s.Type, TypeSelfUpdate); err != nil {
+		return err
+	}
+	if err := requireRID(s.RID); err != nil {
+		return err
+	}
+	if !ValidName(s.Name) {
+		return badRequest("name too long")
+	}
+	if s.DefaultDelayMs != nil && *s.DefaultDelayMs < 0 {
+		return badRequest("default_delay_ms negative")
+	}
+	return nil
+}
+
+// Validate 校验 SelfTalkPassword。
+func (s *SelfTalkPassword) Validate() error {
+	if err := requireVersion(s.V); err != nil {
+		return err
+	}
+	if err := requireType(s.Type, TypeSelfTalkPassword); err != nil {
+		return err
+	}
+	if err := requireRID(s.RID); err != nil {
+		return err
+	}
+	if !ValidTalkPassword(s.TalkPassword) {
+		return badRequest("invalid talk_password")
+	}
+	return nil
+}
+
+// Validate 校验 SelfLoginPassword。
+func (s *SelfLoginPassword) Validate() error {
+	if err := requireVersion(s.V); err != nil {
+		return err
+	}
+	if err := requireType(s.Type, TypeSelfLoginPassword); err != nil {
+		return err
+	}
+	if err := requireRID(s.RID); err != nil {
+		return err
+	}
+	if s.NewPassword == "" {
+		return badRequest("new_password required")
+	}
+	if LoginPasswordForbiddenPrefix(s.NewPassword) {
+		return badRequest("login password must not start with nst_")
+	}
+	if !ValidLoginPassword(s.NewPassword) {
+		return badRequest("invalid new_password")
+	}
+	return nil
+}
+
+// Validate 校验 SelfLogout。
+func (s *SelfLogout) Validate() error {
+	if err := requireVersion(s.V); err != nil {
+		return err
+	}
+	if err := requireType(s.Type, TypeSelfLogout); err != nil {
+		return err
+	}
+	return requireRID(s.RID)
+}
+
+func validateGroupMembers(members []GroupMemberIn) error {
+	for _, m := range members {
+		if !ValidEndpointID(m.ID) {
+			return badRequest("invalid member id")
+		}
+	}
+	return nil
+}
+
+// Validate 校验 GroupCreate。
+func (g *GroupCreate) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupCreate); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if g.ID != "" && !ValidEndpointID(g.ID) {
+		return badRequest("invalid group id")
+	}
+	if !ValidName(g.Name) || g.Name == "" {
+		return badRequest("invalid name")
+	}
+	return validateGroupMembers(g.Members)
+}
+
+// Validate 校验 GroupAdd。
+func (g *GroupAdd) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupAdd); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	if len(g.Members) == 0 {
+		return badRequest("members required")
+	}
+	return validateGroupMembers(g.Members)
+}
+
+// Validate 校验 GroupRemove。
+func (g *GroupRemove) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupRemove); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	if !ValidEndpointID(g.EndpointID) {
+		return badRequest("invalid endpoint_id")
+	}
+	return nil
+}
+
+// Validate 校验 GroupLeave。
+func (g *GroupLeave) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupLeave); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	return nil
+}
+
+// Validate 校验 GroupTransfer。
+func (g *GroupTransfer) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupTransfer); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	if !ValidEndpointID(g.EndpointID) {
+		return badRequest("invalid endpoint_id")
+	}
+	return nil
+}
+
+// Validate 校验 GroupRename。
+func (g *GroupRename) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupRename); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	if !ValidName(g.Name) || g.Name == "" {
+		return badRequest("invalid name")
+	}
+	return nil
+}
+
+// Validate 校验 GroupDissolve。
+func (g *GroupDissolve) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupDissolve); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	return nil
+}
+
+// Validate 校验 GroupList。
+func (g *GroupList) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupList); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	return checkPageLimit(g.Limit)
+}
+
+// Validate 校验 GroupGet。
+func (g *GroupGet) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupGet); err != nil {
+		return err
+	}
+	if err := requireRID(g.RID); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	return checkPageLimit(g.Limit)
+}
+
+// Validate 校验 GroupEvent。
+func (g *GroupEvent) Validate() error {
+	if err := requireVersion(g.V); err != nil {
+		return err
+	}
+	if err := requireType(g.Type, TypeGroupEvent); err != nil {
+		return err
+	}
+	if !ValidEndpointID(g.GroupID) {
+		return badRequest("invalid group_id")
+	}
+	if g.Event == "" {
+		return badRequest("missing event")
+	}
+	if g.EndpointID != "" && !ValidEndpointID(g.EndpointID) {
+		return badRequest("invalid endpoint_id")
+	}
+	return nil
+}
+
+// Validate 校验 Fatal。
+func (f *Fatal) Validate() error {
+	if err := requireVersion(f.V); err != nil {
+		return err
+	}
+	if err := requireType(f.Type, TypeFatal); err != nil {
+		return err
+	}
+	switch f.Reason {
+	case "disabled", "deleted", "password_reset", "protocol":
+		return nil
+	default:
+		return badRequest("invalid reason")
+	}
+}
+
+// Validate 校验 RegisterRequest。
+func (r *RegisterRequest) Validate() error {
+	if r.ID != "" && !ValidEndpointID(r.ID) {
+		return badRequest("invalid id")
+	}
+	if LoginPasswordForbiddenPrefix(r.LoginPassword) {
+		return badRequest("login password must not start with nst_")
+	}
+	if !ValidLoginPassword(r.LoginPassword) {
+		return badRequest("invalid login_password")
+	}
+	if !ValidName(r.Name) {
+		return badRequest("name too long")
+	}
+	if !ValidTalkPassword(r.TalkPassword) {
+		return badRequest("invalid talk_password")
+	}
+	return nil
+}
+
+// MetaCanonicalJSON 将 meta 规范化后序列化（键排序；嵌套 map 同样处理）。
+// encoding/json 对 map[string]T 会按键名字典序输出，因此插入顺序不影响结果。
+func MetaCanonicalJSON(meta map[string]any) ([]byte, error) {
+	if meta == nil {
+		return []byte("{}"), nil
+	}
+	return Marshal(normalizeMetaValue(meta))
+}
+
+func normalizeMetaValue(v any) any {
+	switch t := v.(type) {
+	case map[string]any:
+		out := make(map[string]any, len(t))
+		for k, child := range t {
+			out[k] = normalizeMetaValue(child)
+		}
+		return out
+	case json.Number:
+		return t
+	default:
+		return v
+	}
+}